Published byStanford Medicine

Each year, more 1.5 million unique visitors log on to the Understanding Genetics website, a collaboration between Stanford’s Department of Genetics and the Tech Museum of Innovation in San Jose, to submit questions ranging from genetic testing to differences in siblings’ appearances. The popularity of the site inspired researchers to compile some of the most interesting and important questions into a book version.

Answering people’s questions… makes me think more deeply about how some things happen. For example, there has been a lot of talk for the last few years focused on the fact that chimps have 48 chromosomes and humans have 46. Many intelligent design people use this fact as an example of why evolution isn’t real because they can’t imagine how something like that could happen. After all, extra or missing chromosomes leads to problems like down syndrome, and besides, two species with different numbers of chromosomes can’t successfully breed (think donkeys, horses and their sterile offspring, mules).

This made me dig deeper into the science of chromosome fusion and to think about how something like that might spread. I found out that around 1 in 1,000 people have a balanced translocation where one chromosome is stuck to another. These folks are perfectly normal except they suffer from high rates of miscarriage. Now if two people with the same balanced translocation have kids together, one possibility is someone with the same chromosomes fused together. Now you have someone with the same amount of DNA just parsed out into a smaller number of chromosomes. Then you just need to propose a catastrophe where the survivors are mostly these new folks and voila, the species has a new number of chromosomes.

After we published this scenario on the website, a physician from China contacted us about a man in his practice who had 44 chromosomes because of something very similar to what I described.